There has been known a technique in which a mat of an insulating material is arranged between a carrier of a catalyst which is electrically energized to generate heat, and a case which contains therein the carrier of the catalyst (for example, see a first patent document). According to this mat, when the carrier of the catalyst is electrically energized, it is possible to suppress an electric current from flowing to the case. Incidentally, immediately after engine starting, etc., the temperature of electrodes is low, and hence, when an exhaust gas which has passed through the mat comes in contact with the electrodes, water or moisture in the exhaust gas may condense to the electrodes. Due to this water, insulation resistance between the electrodes and the case will decrease, so there is a fear that electricity may flow to the case from the electrode.